The present invention relates to a block gain equalizer for adjusting gain in an optical transmission system, and more particularly to a block gain equalizer suitable for being inserted in a relaying section of an optical fiber transmission path to adjust gain of an optical signal.
Optical transmission systems often transmit a large quantity of signals by wavelength multiplexing an optical signal to transmit through an optical fiber transmission path.
FIG. 17 represents a principal part of an optical transmission system in which optical fiber transmission paths are connected together. A plurality of optical fiber transmission paths 1011, 1012, . . . 101N have optical repeaters 1021, 1022, . . . 102Nxe2x88x921 disposed among those to amplify a reduced optical signal. In such an optical transmission system, the respective optical fiber transmission paths 1011, 1012, . . . 101N do not uniformly reduce the gain with distance for all frequencies used for optical signals, but a rate of the reduction differs depending upon the frequency. This also applies to a case of each optical repeater 1021, 1022, . . . 102Nxe2x88x921, and Raman amplification causes ununiformity in terms of frequency in an amplification factor. When it is approximately considered that attenuation or amplification of these frequencies occurs at a predetermined slope, it can be regarded that the respective optical fiber transmission paths 1011, 1012, . . . 101N have loss slopes, and the respective optical repeaters 1021, 1022, . . . 102Nxe2x88x921 have gain slopes. In other words, one optical transmission system has a frequency characteristic determined by a sum of loss slopes due to the respective optical fiber transmission paths 1011, 1012, . . . 101N and gain slopes due to the respective optical repeaters 1021, 1022, . . . 102Nxe2x88x921.
The optical transmission system is designed so as to flatten the gain of the entire system in consideration of these loss slopes and gain slopes. Actually, however, the characteristics of the optical fiber transmission paths 1011, 1012, . . . 101N and the optical repeaters 1021, 1022, . . . 102Nxe2x88x921 may be different from what has been expected due to variations in their manufacture and the like. When this xe2x80x9cdeviationxe2x80x9d exceeds a certain tolerance, the system called xe2x80x9ctransmission pathxe2x80x9d does not hold. Thus, in order to prevent such a matter, the block gain equalizer is inserted in the transmission path for each predetermined relaying section in the optical transmission system. Thus, the gains for the respective frequencies are flattened by these block gain equalizers.
FIG. 18 illustrates the state in descriptively. As an example here, a block gain equalizer 103 is disposed behind an optical repeater 102Nxe2x88x921 at the last stage shown in FIG. 17. In this example, the frequency characteristic of the first optical fiber transmission path 1011 is a flat characteristic with respective frequencies having equal signal levels as shown as the first characteristic measurement result 1041. If the frequency characteristic of the optical fiber transmission path 101N behind the optical repeater 102Nxe2x88x921 at the last stage inclines by a large amount depending upon the frequency as shown as the N-th characteristic measurement result 104N, the block gain equalizer 103 is disposed behind it. This block gain equalizer 103 has such a frequency characteristic 105 as to offset the gain slope corresponding to a change in the frequency. Accordingly, the characteristic measurement result 106 of the optical signal after passage of this block gain equalizer 103 is flattened with respect to the frequency.
FIG. 19 represents a circuit configuration of a block gain equalizer for controlling such a gain. This block gain equalizer 103 has an optical coupler 112 which branches an optical signal 111 inputted from the optical fiber transmission path 101N. One optical signal 113 branched by the optical coupler 112 is inputted to a variable gain equalizer 114, where the gain is flattened, and thereafter, it is transmitted to an optical fiber 116 at the following stage as the optical signal 115.
On the other hand, the other optical signal 117 branched by the optical coupler 112 is inputted to a band pass filter (BPF) 118, where only optical signals of a predetermined band are selected. Assuming the optical signals inputted into the block gain equalizer 103 are constituted by signals having respective wavelengths as shown at (a) in FIG. 19 (differences in intensity of signal level are not indicated here), a specific wavelength xcexSV of those wavelengths is used as a supervisory signal. The band pass filter 118 selects the supervisory signal 119 having this wavelength xcexSV. (b) in FIG. 19 shows a state in which only the supervisory signal 119 has passed through the band pass filter 118.
The supervisory signal 119 thus selected is inputted into a photodiode (PD) 121, and is photoelectric-converted into an electric signal 122. This electric signal 122 representing the supervisory signal 119 is inputted into a gain control circuit 123, which reads data on loss slope previously incorporated in the supervisory signal 119 to input, into the variable gain equalizer 114, such a voltage signal 125 as to become a loss slope designated. As a result, the gain of the optical signal 115 (see FIG. 19(c)) obtained is flattened with respect to the used frequency band. Of course, this data on the loss slope may be data transmitted to this block gain equalizer 103 through other paths.
FIG. 20 represents an example of a characteristic of the conventional variable gain equalizer. The variable gain equalizer 114 is adapted such that the loss slope relative to wavelength changes in response to the voltage level of the voltage signal 125. In this figure, when the voltage signal 125 indicates 3V (volt), the frequency characteristic is flat as indicated by solid line, and no correction is performed. When the voltage signal 125 indicates 2V, the variable gain equalizer has such a characteristic that the more the wavelength xcex is increased, the less becomes the loss. When the voltage signal 125 indicates 4V, the variable gain equalizer has such a characteristic that the more the wavelength xcex is increased, the more becomes the loss. Therefore, the variable gain equalizer 114 is capable of adjusting the gain of the optical signal 113 in response to the voltage signal 125 to output an optical signal 115, which has become flat with respect to the frequency.
The variable gain equalizer 114 shown in FIG. 19 flattens the gain by changing the attenuation factor relative to the frequency of an optical signal inputted. Therefore, the optical signal inputted into the block gain equalizer 103 does not increase the signal level although the signal level may be reduced. For this reason, apart from a case where the block gain equalizer 103 is inserted at the last stage of the transmission path, when it is inserted in a section along the path, the length of the optical fibers in the section has to be made shorter than other sections. This leads to a problem that the optical transmission system becomes expensive.
It is an object of the present invention to provide a block gain equalizer which controls an amount of gain slope and does not cause any loss in optical signal.
According to the invention specified in claim 1, there is provided a block gain equalizer having: (a) doped fibers added with rare earth elements for receiving an optical signal transmitted from a transmission path on the upstream side and relaying to a transmission path on the downstream side; (b) a pumping source for injecting pumping light into the doped fibers; and (c) power setting means for setting power of the pumping light to be outputted by this pumping source to power, which corresponds to gain characteristic for correcting the frequency characteristic of the optical signal transmitted from the transmission path.
More specifically, according to the invention specified in claim 1, focusing attention to the fact that when the power of the pumping light to be injected into the doped fibers added with rare earth elements is appropriately adjusted, the frequency characteristic of the gain due to the doped fibers changes, a block gain equalizer to be disposed in the transmission path in order to equalize the gain is caused to contain the doped fibers, and the power of the pumping source is set to power which corresponds to gain characteristic for correcting the frequency characteristic of an optical signal. Thereby, such gain loss as when equalizing is performed by an attenuator does not occur.
According to the invention specified in claim 2, there is provided a block gain equalizer having: (a) doped fibers added with rare earth elements for receiving an optical signal transmitted from a transmission path on the upstream side and relaying to a transmission path on the downstream side; (b) a pumping source for injecting pumping light into the doped fibers; (c) power setting means for setting power of the pumping light outputted by this pumping source to power which corresponds to a gain characteristic for correcting the frequency characteristic of the optical signal transmitted from the transmission path; and (d) an attenuator for attenuating, by a predetermined amount, the optical signal amplified by the doped fibers by the use of the pumping light of the power set by this power setting means.
More specifically, according to the invention specified in claim 2, focusing attention to the fact that when the power of the pumping light to be injected into the doped fibers added with rare earth elements is appropriately adjusted, the frequency characteristic of the gain due to the doped fibers changes, a block gain equalizer to be disposed in the transmission path in order to equalize the gain is caused to contain the doped fibers, and the power of the pumping source is set to power which corresponds to gain characteristic for correcting the frequency characteristic of an optical signal. Thereby, such gain loss as when equalizing is performed by an attenuator does not occur. Since, however, the doped fibers are used, there are cases where the gain is increased conversely and it is judged to be undesirable to transmit an optical signal through a transmission path having a predetermined length. Thus, according to the invention specified in claim 2, there is prepared an attenuator for attenuating, by a predetermined amount, an optical signal obtained by amplifying by the doped fibers. If the attenuation for the amount thus amplified is not required, the invention specified in claim 1 will be applied.
According to the invention specified in claim 3, there is provided a block gain equalizer having: (a) doped fibers added with rare earth elements for receiving an optical signal transmitted from a transmission path on the upstream side and relaying to a transmission path on the downstream side; (b) a pumping source for injecting pumping light into the doped fibers; (c) characteristic storing means in which there is stored, in advance, the frequency characteristic of an amplification factor relative to various values of power in injecting the pumping light into the doped fibers; (d) power setting means for setting power of the pumping light which corresponds to desired frequency characteristic stored in this characteristic storing means on the basis of data on an instruction to flatten the frequency characteristic of the optical signal; and (e) an attenuator for attenuating, when the amplification factor of the optical signal amplified by the doped fibers by the use of the pumping light of power set by this power setting means is higher than a desired value, this amplification factor to the desired value.
More specifically, according to the invention specified in claim 3, focusing attention to the fact that when the power of the pumping light to be injected into the doped fibers added with rare earth elements is appropriately adjusted, the frequency characteristic of the gain due to the doped fibers changes, a block gain equalizer to be disposed in the transmission path in order to equalize the gain is caused to contain the doped fibers, and the power of the pumping source is set to power which corresponds to gain characteristic for correcting the frequency characteristic of an optical signal. For this reason, according to the invention specified in claim 3, there is prepared characteristic storing means in which there has been stored, in advance, the frequency characteristic of amplification factor relative to various values of power in injecting the pumping light into the doped fibers, and there is to be set power of pumping light which corresponds to desired frequency characteristic stored in this characteristic storing means on the basis of data on an instruction to flatten the frequency characteristic of the optical signal. Even in the invention specified in claim 3, the amplification factor of an optical signal obtained by amplifying by the doped fibers by the use of the pumping light may be higher than a desired value. In such a case, there is used an attenuator for attenuating this to the desired value.
According to the invention specified in claim 4, there is provided a block gain equalizer having: (a) doped fibers added with rare earth elements for receiving an optical signal transmitted from a transmission path on the upstream side and relaying to a transmission path on the downstream side; (b) a pumping source for injecting pumping light to the doped fibers; (c) characteristic storing means in which there are stored, in advance, the frequency characteristics of amplification factor relative to various values of power in injecting the pumping light into the doped fibers; (d) characteristic distinguishing means for distinguishing the frequency characteristic of the optical signal; (e) power setting means for selecting an optimum frequency characteristic to flatten the frequency of a characteristic distinguished from among frequency characteristics stored in the characteristic storing means by this characteristic distinguishing means to set power of the pumping light corresponding thereto; and (f) an attenuator for attenuating, when the amplification factor of the optical signal amplified by the doped fibers by the use of the pumping light of power set by this power setting means is higher than a desired value, this amplification factor to the desired value.
More specifically, according to the invention specified in claim 4, focusing attention to the fact that when the power of the pumping light to be injected into the doped fibers added with rare earth elements is appropriately adjusted, the frequency characteristic of the gain due to the doped fibers changes, a block gain equalizer to be disposed in the transmission path in order to equalize the gain is caused to contain the doped fibers, and the power of the pumping source is set to power which corresponds to gain characteristic for correcting the frequency characteristic of an optical signal. For this reason, according to the invention specified in claim 4, there is prepared characteristic storing means in which there has been stored, in advance, the frequency characteristic of amplification factor relative to various values of power in injecting the pumping light into the doped fibers, and there is provided characteristic distinguishing means for distinguishing the frequency characteristic of an optical signal inputted, and by selecting an optimum frequency characteristic to flatten the frequency of a characteristic obtained by distinguishing from among frequency characteristics stored in the characteristic storing means by this characteristic discriminating means to set the power of pumping light corresponding thereto. Even in the invention specified in claim 4, the amplification factor of an optical signal obtained by amplifying by the doped fibers by the use of the pumping light having power set by this power setting means may be higher than a desired value. In such a case, there is used an attenuator for attenuating this amplification factor to the desired value.
The invention specified in claim 5 is characterized in that the pumping source pumps the doped fibers in accordance with a backward pumping system in a block gain equalizer according to claim 1 to claim 4.
Thereby, it is possible to realize a high-efficiency block gain equalizer having low power consumption.
The invention specified in claim 6 is characterized in that the pumping source pumps the doped fibers in accordance with the forward pumping system in a block gain equalizer according to claims 1 to 4.
This enables a block gain equalizer having a less amount of worsened noise factor to be realized even if the input light intensity is increased.
The invention according to claim 7 is characterized in that the pumping source consists of a light source for the backward pumping system and a light source for the forward pumping system in a block gain equalizer according to claims 1 to 4, and one of these light sources is selected under predetermined conditions to pump the doped fibers.
As described above, according to the invention specified in claim 7, the backward pumping system and the forward pumping system are properly used, and therefore, the optical transmission system can be kept in a better condition by properly using the high-efficiency backward pumping system with low power consumption and the forward pumping system which has a less amount of worsened noise factor even though the input light intensity is increased.